1. Field of the Invention
The present invention relates to a process of electrographic reproduction on an arbitrary support, with the aid of a magnetic, monocomponent developing powder. More particularly, it relates to a process of electrographic reproduction in which an image of electrostatic charges formed on an intermediate support, such as a photoconductor or any other surface capable of retaining an image of electrostatic charges, is developed with the aid of a magnetic, monocomponent developing powder to form a powder image which is transferred by the action of electrical means (e.g., an electric field, corona discharge device, etc.) to an arbitrary support, with the image thus obtained being fixed on the support by pressure or heat.
2. Description of the Prior Art
Electrographic document copying processes, particularly those using ordinary paper, have been refined to a substantial degree over the past ten years.
Basically in these processes, a uniform charge is produced on a photoconductor with the aid of a corona effect device (hereinafter called a "corona device"). By selective exposure starting with an original, an image of charges is produced which is subsequently developed with the aid of a developing powder.
There are a number of types of known developing powders of which the following are noteworthy:
(1) Developing powders with two components (hereinafter called "bicomponent powders") which use two types of particles, the vehicle (or "carrier") and the developer (or "toner"). The vehicle generally comprises glass microspheres or the like which have a large diameter as compared to the developer particles. The developer particles are held on the surface of the vehicle particles by triboelectricity and are based on carbon black surrounded by, i.e., coated with resin; and
(2) Monocomponent developing powders which employ only a single type of particles, generally magnetic particles coated with appropriate resins; depending on the conditions of manufacture these particles have a more or less conducting nature.
The development of the image of electrostatic charges with the aid of a bicomponent developing powder may be effected by various processes, of which currently the most widely used are:
(1) The so-called "cascade process," such as described in U.S. Pat. No. 2,618,552, in which the developer particles are deposited on the charges of the latent image having opposite sign to the charges borne by said particles, by means of electrostatic attraction. The powder images thus developed are generally easily transferrable under the action of an external electric field or under the action of a "corona"; and
(2) The developing process employing a magnetic brush, such as described in U.S. Pat. No. 2,874,063, in which the vehicle particles comprise smooth iron filings which may be coated with a triboelectric resin. The developer particles are generally held onto the vehicle particles by triboelectricity.
In the two processes wherein bicomponent developing powders are used, it is essential that the developer particles are insulated to a high degree so as to retain their charge needed for good development of the image. In this connection such development processes employing bicomponent developing powders have certain disadvantages. Since the developer particles are held electrostatically to the surface of the vehicle, it is essential that the two elements are provided in the correct ratio. If there is an excess of developer particles they are no longer sufficiently retained by the vehicle particles but are dispersed in the machine, causing significant soiling of said machine, particularly regarding the optical system. This necessitates frequent cleaning of the machine, giving rise to major maintenance costs for this type of machine.
Certain automatic dosing devices for maintaining the quantity of developer have been employed. These devices add developer after each copy, but they are not entirely satisfactory since the quantity of developer used for the development of a copy depends essentially on the nature of the original--in particular, the amount of black therein.
Another disadvantage connected with the use of bicomponent developing powder is the need to mechanically clean the photoconductor after each copy. This cleaning necessitates a complex system of mechanical brushes and of aspiration, which are also sources of contamination of the apparatus (in contrast to cleaning systems where a monocomponent magnetic toner is employed, in which case a magnetic brush is used for the cleaning).
For these reasons in recent years electrostatic copying machines have been developed which employ monocomponent magnetic developing powders of a more or less conducting nature. In general, to develop an image of electrostatic charges using monocomponent developing powders, a magnetic brush is used comprising a metallic cylinder inside which magnets may be revolved, which cylinder is covered with a layer of monocomponent magnetic developing powders. These powders generally have a more or less conducting nature, and are charged by induction with the approach of the image of charges to the developer; the development may be promoted by the presence of an external electric field.
It should be noted that the use of monocomponent developing powders is not a development solely in the realm of powders having a magnetic character. There are also nonmagnetic monocomponent-type powders, such as those described in Fr. Pat. No. 2,362,428; and the present invention applies as well to such systems.
Monocomponent developing powders, and in particular those of a magnetic character, have the advantage of not soiling the machine in which they are used, since they are held securely on the magnetic brush. Such powders are currently widely used in so-called "direct" processes, wherein, for example, they are used for the development of photoconducting zinc oxide papers. In this application they are completely satisfactory. However, this direct process is only used on low-output machines, because it is more economical to employ machines using plain paper when one intends to make a large number of copies, i.e., more than 3000 to 5000 copies per month. Also, users tend to prefer copies on an ordinary support.
However, thus far, despite numerous attempts, it has been impossible to obtain copies of good quality with images of monocomponent developing powder transferred to an arbitrary support, in particular to plain paper. Worthy of note is the fact that current commercial machines employing monocomponent developing powders to form a powder image on a so-called "plain paper" support do not in fact use ordinary paper but a treated paper having a low surface conductivity. The use of true plain paper in such machines yields images of only mediocre quality which are defective in clarity and definition, and this mediocrity is aggravated in the event of high ambient humidity. The present inventors believe that electrical microdischarges are produced during the transfer of the powder image, with the phenomenon being intensified or amplified in the event of increased surface conductivity of the support, notably due to excessive humidity. Furthermore, this phenomenon is more significant as a result of the fact that the developing powder has low surface resistivity. This is because the current machines which use these monocomponent developing powders employ treated paper which is covered with a resin coating which makes its surface conductivity low. Among such treatments one might note, in particular, that described in U.S. Pat. No. 4,199,356, according to which a viscous liquid is applied, e.g., a silicone oil, to the paper which is to receive the final image. Such a process, in addition to the fact that it is applied only to a final receptor paper and not to intermediate organs and therefore does not protect the latter, leaves an oily insulating deposit on the paper. Such paper is much more costly than plain paper, and/or it has an inferior appearance. This necessitates maintaining a special stock, and therefore the user cannot always use the support of his choice.
As a response to variations in conductivity connected with atmospheric conditions it has been proposed to use plain paper which has been pre-dried. This necessitates special equipment in the machine, and it still does not completely solve the problem. In practice it has been found that, although the quality of the copy is improved, an "explosion" effect is observed in the image, amounting to a lack of clarity of outlines, which may render the image illegible when sharpness of definition is the deciding factor.
Other solutions have been tried with regard to monocomponent developing powders. For example, an attempt has been made to reduce the conductivity of the powder by using an increased amount of the appropriate coating resins. This appreciably increased the quality of the transferred image, but the development speed decreased abruptly, which is unacceptable. This is explained by the fact that since an electrical charge is placed on the developing powder by induction due to the presence of the charges in the charged image on the intermediate organ, the speed of the charging of the powder depends on the time constant RC of the powder particles. If the resistivity of the powder is increased, the polarization time for induction on the powder particles at the approach of the charges in the charged image on the intermediate organ increases correspondingly and quickly becomes excessive, especially in relation to the development time of the charged image. As a result the development speed of the charged image must be reduced, and correspondingly the speed of movement of the paper must also be reduced. Consequently, with a system using this type of developing powder one cannot achieve a high enough copying rate to make the machine competitive.
None of the solutions proposed heretofore actually permits the use of monocomponent developing powder in a machine where the powder image is transferred to a plain paper support such that a good quality image is obtained thereon (i.e., good contrast, no "explosion," no edge effects, etc.), without limiting the development speed of the image and without regard to the atmospheric conditions.